High-pressure metal vapor discharge tube



Dec. 30, 1941 W. ELENBAAS HIGH-PRESSURE METAL VAPOR DISCHARGE TUBE FiledMay 6, 1940 Inven'tor: 'Mem Lenbaas,

His A't'torneg.

Patented Dec. 30, 1941 HIGH-PRESSURE METAL VAPOR DISCHARGE TUBE I WillemElenbaas, Eindhoven, Netherlands,

assignor to General Electric Company Application May 6, 1940, Serial No.333,653 In the Netherlands April 1, 1939 Claims.

It is known that in high-pressure metal vapor discharge tubes thepressure of the metal vapor greatly increases upon ignition of the tubeand this increase is attended by an increase in the operating voltage. Ahigh-pressure metal vapor discharge tube therefore exhibits the propertythat when the tube is in normal use the operating voltage issubstantially higher than (at least twice as high as) the operatingvoltage which occurs directly upon initiation of the arc discharge. Theincrease in the vapor pressure is also attended by the phenomenon thatthe discharge contracts, that is to say no longer fills the wholesection of the discharge tube.

It is known to limit the quantity of vaporizable metal in thesedischarge tubes to such an extent that when the tube is in normal usethis quantity of metal is entirely vaporized and the metal vapor isunsaturated. In the presence of a plentiful supply of vaporizable metalthe vapor pressure and thus the operating voltage of the discharge willalways increase when the heating of the discharge tube increases. Bylimiting the quantity of metal in the manner indicated it is ensuredthat from the moment when all the metal has passed into the vapor statethe vapor pressure can only rise slowly when the heating of thedischarge tube is continued so that the operating voltage also increasesvery slowly. This is very advantageous because a, more stable operationis thus obtained, as becomes manifest, for example, in that uponvariation in the condition of the medium to which the heat istransmitted, that is to say in the case of variations in the heatdissipation and also in the case of variations of the voltage of thesource of supply current, the operating voltage of the discharge remainssubstantially constant.

When using such a limited quantity of vaporizable metal there is alsothe possibility of choosing the electric load of the discharge tube sothat the metal vapor is superheated to a great extent in the normal useof the tube, that is to say the lowest temperature of the dischargespace is substantially higher than the temperature at which thesaturation vapor pressure is equal to the pressure of the unsaturatedmetal vapor in the discharge tube. This offers the advantage ofshortening the heating-up period of the discharge tube, i. e. the timenecessary to bring the discharge tube from the cold condition into thecondition of normal operation.

The invention relates to high-pressure metal vapor discharge tubeshaving such a limited is in normal use all the metal is vaporized and ithas for its object to improve the said tubes. In the use of thesedischarge tubes the unpleasant phenomenon occurs that when the tube isin normal use and the discharge is interrupted (either deliberately byinterruption of the circuit or unintentionally due to the fact that, forexample, the voltage of the source of current momentarily falls belowthe voltage necessary for the maintenance of the discharge) the voltagenecessary for reinitiation of the discharge is substantially higher thanthe starting-up voltage when the discharge tube is in the coldcondition. In normal cases the voltage available is therefore too low tobring about immediate reignition. This is due to the high-vapor pressurewhich prevails in the tube on the discharge being interrupted.Re-ignition is not possible in this case until the discharge tube hascooled to such an extent that the vapor pressure has greatly fallen.This requires a certain time period; the more the metal vapor isunsaturated the longer is this cooling period since the discharge tubehas first to be subjected to an intenser cooling before the metal vaporstarts to condense.

It has been suggested to shorten the cooling period of high-pressuremet-a1 vapor discharge tubes which are not artificially cooled duringnormal use by cooling a part of the tube wall by means of a. flow ofliquid directly upon the interruption of the discharge. The contrivancesrequired for this purpose, however, involve a great complicationparticularly if the arrangement is such that upon interruption of thedischarge artificial cooling starts automatically. If use is not made ofan automatically operating device the discharge tube must be watchedcontinually.

According to the invention the discharge tube is provided with anauxiliary receptacle which is heated by a heating element connected inseries with the discharge path, the thermal capacity of the saidreceptacle being made so low and its heat dissipating capacity beingmade so high that when the discharge is interrupted while the tube is innormal use the temperature in the auxiliary receptacle decreases morerapidly than the lowest temperature in the discharge space proper, theheating element being in addition so proportioned that onthe dischargetube being put into service the auxiliary receptacle is caused to assumea higher temperature so rapidly that ,the heating-up period of thedischarge tube is not longer than in the absence of the auxiliaryquantity of vaporizable metal that when the tube receptacle.

Due to the fact that the temperature in the auxiliary receptacle fallsmore rapidly than in the discharge space proper a more rapidcondensation of the metal vapor ensues in this auxiliary receptacleafter interruption of the discharge and the cooling period necessary forreducing the vapor pressure to such extent that the discharge canre-ignite becomes shorter than in the absence of the auxiliaryreceptacle. It is known that, a short time after initiation of thedischarge, the current strength is higher than when the tube is innormal use. Consequently, this large current, which also flows throughthe heating element brings about a rapid heatingup of the auxiliaryreceptacle with the result that the metal condensed therein is vaporizedagain. By making the capacity of the heating element suificiently highit may be ensured that the heating-up period of the discharge tube, thatis to say the time required for bringing the tube from the coldcondition into the normal operating condition, is not increased by theprovision of the auxiliary receptacle. Obviously, the heating elementmust also be proportioned in such manner that when the discharge tube isin normal use the temperature in the auxiliary receptacle is not lowerthan the temperature at which the desired vapor pressure is obtained.

'The thermal capacity of the auxiliary receptacle can be reduced bydecreasing the wall thickness and the width thereof. Simultaneousdecrease of the wall thickness and of the width permits of highpressures being nevertheless admitted in the discharge tube. Theexternal diameter of the auxiliary receptacle is preferably less thanhalf the external diameter of the discharge tube.

When the auxiliary receptacle is enclosed in a gaseous atmosphere asmall diameter is also favorable as regards dissipation of heat sincethe smaller the diameter the higher is the dissipation of heat per unitof wall surface. The dissipation of heat is also influenced by thepressure and the nature of the surrounding gaseous atmosphere andincreases as the pressure and the heat conduction of the gas increase.In order to intensify the dissipation of heat by radiation it may beadvisable to blacken the outside of the wall of the auxiliaryreceptacle.

The wall of the auxiliary receptacle has preferably one or more metalbodies, for example metal wires, led through it. Due to the heatconduction by these bodies the heat dissipating capacity of theauxiliary receptacle is increased, it being preferable for the saidbodies to have a large surface area outside the auxiliary receptacle andin some circumstances, to be black- .ened. For this purpose, forexample, a small metal plate may be fixed to a wire led through thewall.

As an alternative, a heat-radiating body of low thermal capacity, forexample a thin metal plate, may be housed in the auxiliary receptacle.In the cooling period this body dissipates its heat rapidly by radiationso that it very rapidly assumes a lower temperature than the wall of theauxiliary receptacle and the metal vapor is thus condensed on the saidbody.

The heating element is preferably so constructed as to leave free alarge part, for example more than 75%, of the wall of the auxiliaryrecept'acle so that this part can dissipate its heat directly to thesurroundings.

In order that the invention may be clearly understood and readilycarried into effect it will now be described more fully, with referenceto the accompanying drawing in which the single figure illustrates ahigh-pressure vapor tube embodying my invention.

Referring to the single figure of the drawing, l designates acylindrical discharge tube made of quartz and having an internal and anexternal diameter of 7.5 and 10 mms. respectively. This discharge tubecomprises two incandescent electrodes 2 and 3 which are provided with amaterial capable of copiously emitting electrons and are heated to therequired 'high temperature solely by the discharge. The spacing betweenthe electrodes is 22 mms. The gas-filling of the tube is constituted byargon at a pressure of about 2 cms. to which a small quantity of mercuryis added.

The bottom end of the discharge tube I has sealed to it an auxiliaryreceptacle 4 constituted by a small cylindrical tube of quartz having aninternal and an external diameter of 2 and 2.5 mms. respectively. Thelength of this auxiliary receptacle is about 10 mms. The closed end ofthis auxiliary receptacle has a tungsten wire 5 of 400 microns thicknesssealed in it. The auxiliary receptacle contains a small molybdenum plate6 which is fixed to the said tungsten wire and is about 4 mms. in lengthand almost 2 mms.

in width. In order to reduce the thermal capacity of this plate thethickness is made very low; it is 18 microns. In order to increase heatradiation the plate 6 is blackened. Outside the auxiliary receptacle thewire 5 has fixed to it a blackened nickel plate 1 which has a thicknessof microns and is 20 mms. in length and width. The auxiliary receptacle4 is surrounded by a heating element 8 constituted by 10 turns of anickel-chromium wire (diameter microns). One end of this heating elementis connected through a wire 9 to the electrode 3, whereas the other endis connected through wire H] to the leading-in wire II which is leadthrough the pinch i2 of the bulb l3 and is connected to the contactmember [4 of the cap I 5. This pinch has also two supporting wires l6and" I1 secured in it to carry the discharge tube 1, the wire I! alsoserving as a current-supply member for the electrode 2 and beingconnected to the contact member IS. The space between the discharge tubeand the bulb I3 is filled with nitrogen at a pressure of 50 cms.

The lamp is used for the emission of rays, for example for illuminationor radiation by ultraviolet rays, 'and is supplied from an alternatingcurrent supply circuit of 220 volts with the interposition of a chokecoil. When the tube is in use it exhibits a high-pressure mercury vapordischarge at which the absorption of energy in the discharge tube is 75watts and the strength of the discharge current is 0.75 ampere. Thequantity of mercury introduced into the discharge tube is so low thatthe operating voltage of the discharge when the tube is in normal use is.120 volts. In this case the mercury vapor is superheated.

When the above discharge tube was tested and was cut out of circuitwhile in normal operation the time in which the vapor pressure fell tosuch an extent that a discharge was re-initiated amounted to 50 seconds.When the auxiliary receptacle was not provided and the constructionremained otherwise unaltered this time period was 95 seconds. Theinvention consequently ensured a considerable shortening of this timeperiod.

by way of example,

What I claim is:

1. A high-pressure metal vapor discharge tube comprising a sealedenvelope containing a pair of electrodes and a gaseous atmospherecomprising a vaporizable metal in an amount so small that it iscompletely vaporized during operation or the tube, an auxiliaryreceptacle extending from said envelope and communicating with theinterior thereof, a heating element connected in series with thedischarge path between said electrodes and located adjacent to saidreceptacle so as to supply appreciably more heat thereto than to theenvelope proper, the thermal capacity of said receptacle being so lowandits heat dissipation being so high that upon interruption of thedischarge the receptacle cools more rapidly than any portion of saidenvelope, the said heating element serving to heat the said receptacle,upon restarting of the discharge in the tube, so that the restartingtime is not longer that it would be in the absence of the saidreceptacle. 2. A discharge tube as set forth in claim 1 wherein theauxiliary receptacle is provided with a metallic conductor memberextending through the end thereof and serving to increase the heatdissipating capacity of the receptacle.

3. A discharge tube as set forth in claim 1 wherein the auxiliaryreceptacle contains a thin metallic conductor member which serves,during the cooling period of the tube, to rapidly dissipate heat byradiation so that the vaporizable material is condensed thereon.

4. A discharge tube as set forth in claim 1 wherein the auxiliaryreceptacle is provided with a metallic conductor member extendingthrough the end thereof and having a large surface area outside saidreceptacle thereby serving to increase the heat dissipating capacity ofthe said receptacle.

5. A discharge tube as set forth in claim 1 wherein the auxiliaryreceptacle contains a thin metallic conductor member which serves,during the cooling period or the tube, to rapidly dissipate heat byradiation so that the vaporizable metal is condensed thereon and ametallic conductor member connected to said thin conductor member andextending through the end of the receptacle and having a large surfacearea outside the receptacle thereby increasing the heat dissipatingcapacity of said receptacle.

' WILLEM ELENBAAS

